Conventionally, in disposable diapers comprising front and rear waist regions and a crotch region extending between these front and rear waist regions, it is known to join the front and rear waist regions together by seam arrays along the respective opposite side edges of the diapers. For example, PATENT DOCUMENT 1 (JP 2005-205026 A) discloses an apparatus by which the front and rear waist regions are put flat together and are nipped along respective opposite side edges by a nipper unit and then sonic-sealed together.
In such an apparatus, a web comprising continuously arranged disposable diapers is transported along an outer peripheral surface of a drum. Ultrasonic wave units projecting out from the inside of the drum cooperate with anvils coming in contact with the associated ultrasonic wave units from the outside of the drum to form seal arrays along which the side edges of the diaper are joined together. Specifically, respective pairs of the ultrasonic wave units and the anvils cooperate with each other to form the seal arrays and six (6) pairs of ultrasonic wave units and anvils are arranged along the outer periphery of the drum at regular intervals so that the seal arrays may be continuously formed. The surface of the anvil is formed with a plurality of protrusions so that the side edges of the diaper may be formed with the seal arrays in the same pattern as the pattern of the protrusions.